1. Field of the Invention
The invention relates to a multiple-ratio power transmission mechanism in a hybrid vehicle powertrain.
2. Background Art
An example of a hybrid electric vehicle powertrain is disclosed in U.S. Pat. No. 6,585,066, which is assigned to the assignee of the present invention. The powertrain includes an electric motor, such as a high voltage induction motor, situated between an internal combustion engine and a multiple-ratio power transmission mechanism, wherein driving torque of the engine is supplemented with electric motor torque. The combined engine torque and electric motor torque is transferred to vehicle traction wheels through multiple-ratio gearing of the power transmission mechanism. A wet clutch assembly may be included in the power flow path between the torque input element of the multiple-ratio power transmission mechanism and the crankshaft of the engine. The torque output side of the clutch is connected to the rotor of the electric motor, which functions as a motor/alternator unit. The engine and the electric motor are controlled so that the engine and the electric motor function in their respective optimum efficiency operating ranges, thereby improving fuel economy and reducing undesirable exhaust gas emissions from the engine.
During operation from a standing start, the powertrain of the '066 patent receives a power request or command by the vehicle driver for power to effect controlled acceleration. The driver command is in the form of a signal that is a function of the depression of a vehicle accelerator pedal. During a power-on downshift in a powertrain with a conventional automatic transmission, the engine torque will increase because of the advancement of the accelerator pedal. That increase in engine torque is used to both accelerate the vehicle and overcome the engine's inertia as the engine speed increases to the value corresponding to a higher gear ratio.
During the speed ratio change, the transmission remains initially in the upshifted torque ratio. A torque ratio change takes place at the end of the speed ratio change. During the speed ratio change, there is no significant change in wheel torque, which causes a delay in the desired acceleration. The length of the delay depends on the amount of torque that the engine is producing and the amount of the effective inertia mass connected to the engine.
Torque fluctuations in the powertrain can occur also at the end of the downshift as friction torque establishing elements of the transmission are engaged and released synchronously. This also can result in a lack of smoothness of a downshift.